Enhanced toughening of polymeric materials has been a topic of intense research for several decades. Most of the historic effort on toughening polymers has focused on rigid materials. Specifically, on the incorporation of additives or fillers to a polymeric formulation in an effort to develop an approach that is generally applicable to multiple materials at a low cost. The impact of the filler depends on the filler size, shape, loading, and dispersion. The impact of the added filler can be enhanced by functionalizing the filler particle to interact more strongly with the host polymer network. However, it is difficult to obtain an intimate dispersion of the filler particulate in the polymer necessary to optimize the toughening. This difficulty becomes even more pronounced as the filler size is decreased into the nanoscale regime. In addition, these polymeric materials toughened by the addition of filler cannot be implemented into applications that require reduced thicknesses on the order of the filler particle size or, more practically, several times larger than the filler particle size. In addition, the filler particulate is often more dense that the polymer, which can lead to particle settling during cure, thereby leading to non-uniform material properties. Toughening has also been explored in rigid materials through the inclusion of rubbery domains. The rubbery domains are the result of the incorporation of rubber particulate or the phase separation of a rubbery additive to alter fracture mechanics and produce higher toughness. A similar mechanism is attributed for improvements in toughness through the incorporation of hyperbranched polymers that also phase separate from the host polymer. Rubber toughening through phase separation requires a balance of solubilities, polymer processing kinetics, and phase separation kinetics to obtain rubbery domains dispersed throughout the material without producing phase separation. To mitigate solubility and kinetic issues, pre-fabricated rubber particulates can be added into the host polymer but they will exhibit the same drawbacks related to particle incorporation discussed previously.
Transitioning the toughening mechanisms developed for rigid polymers to soft polymeric materials is difficult due to changes in the fracture mechanism and the energy associated with the crack propagation. The incorporation of fillers has been shown to enhance the mechanical properties of soft polymeric materials however the same disadvantages identified in rigid materials are still present. In addition, the incorporation of filler can lead to undesired decreases in the elasticity and elongation at break.
A method utilized to enhance soft polymeric material toughness is to produce a dual polymer network in a swollen polymer gel. While successful, these gels are typically produced through a complicated sequence of reaction conditions. Initially, a relatively rigid, high cross-link density network is formed and then swollen with a solvent to allow for infiltration by a secondary network precursor. The secondary component is then polymerized to produce a second and more flexible low cross-link density network within the more rigid system. The two independent networks are entangled to provide a novel toughening mechanism. As the dual network gel is deformed, the rigid network is fractured but is held together by the more flexible network. While these dual networks enhance the gel strength and toughness, it is not easily scaled or transitioned to other gel systems. Specifically, this toughening mechanism has only been reported for hydrogel systems that utilize water as a solvent. The water-based hydrogels have limited applicability due to the relatively high volatility of water leading to evaporation and a change in the material properties. In addition, the freezing and boiling points, respectively, fall within the operational temperature of many applications and will have a large impact on the material performance.
Therefore, the inventors have provided improved toughened polymeric materials and methods of forming toughened polymeric materials.